1. Field of the Invention
The present invention relates in general to wireless communications systems and, in particular, to system and method for dynamically calibrating base station timing.
2. Description of Related Art and General Background
Calibrating and maintaining proper timing is an important concern in communication systems. This is particularly true in wireless communications operating under Code Division Multiple Access (CDMA) schemes. CDMA is a digital radio-frequency (RF) channelization technique, defined in Telecommunications Industry Association/Electronics Industries Association Interim Standard-95 (TIA/EIA IS-95), entitled xe2x80x9cMOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEMxe2x80x9d, published in 1993. Other aspects of CDMA communication systems are defined in well-known standards, such as, for example, TLA/EIA IS-97, TIA/EIA IS-98, cdmaOne, cdma2000, and wideband CDMA (WCDMA) standards.
Wireless communication systems employing CDMA technologies assign a unique code to communication signals and apply pseudorandom noise (PN) modulation to spread these communication signals across a common wideband spread spectrum bandwidth. In particular, the communication signals are modulated with PN sequences to spread the signals over a wide bandwidth. CDMA systems employ two short PN code sequences (i.e., xe2x80x9cIxe2x80x9d and xe2x80x9cQxe2x80x9d) and one long PN code sequence. The short PN codes are used for quadrature spreading and have unique offsets serving as identifiers for a cell or a sector. At the WD 110 receiver, the received spread spectrum signal is despread in order to recover the original data. As long as the WD 110 receiver has the correct code, it can successfully detect and select its communication signal from the other signals concurrently transmitted over the same bandwidth. The encoding/decoding, modulation/demodulation, and spreading/despreading processes depend on accurate timing for synchronization and proper system operation.
FIG. 1 (Prior Art) illustrates a simplified block diagram of CDMA wireless communication system 100. System 100 allows mobile station or wireless communication device (WD) 110 to communicate with an Interworking Function (IWF) 108 via a base station (BS) 106. The IWF 108 serves as a gateway between the wireless network and other networks, such as the Public Switched Telephone Network (PSTN) and wireline packet data networks providing Internet- or Intranet-based access. WD 110 communicates with BS 106, which is associated with a geographic cell or sector, via the wireless interface Um on the reverse link transmission path. BS 106 is configured to process the communication signals from WD 110.
On the forward link transmission path, BS 106 communicates with WD 110 via the wireless interface Um. During forward link transmissions, each BS 106 is capable of transmitting information-bearing signals as well as control signals, such as pilot signals. Pilot signals are used to identify the BS 106 best suited to accommodate reverse link transmissions. Pilot signals also provide a time and coherent phase reference to enable WD 110 to obtain initial system synchronization and facilitate coherent demodulation on the forward link. All pilot signals are subjected to the same PN spreading code but with a different code phase offsets to enable WD 110 to distinguish between different pilot signals, thereby identifying the originating BS 106.
As noted above, proper CDMA system 100 operation requires accurate timing. For example, in accordance with IS-95 and IS-97 standards, each BS 106 is required to use a time base reference from which all time-sensitive transmission components, including pilot PN code sequences and frames, are to be derived. Each BS 106 time base reference is required to be synchronized to CDMA system time. Benefits of synchronized BSs 106 include, for example, improved hand-off speed and reliability, enhanced initial system acquisition (i.e., cell search) speed, increased handset (e.g. WD 110) stand-by time, and improved reliability and power economy due to common channel hand-off operations.
CDMA system time may employ a Global Positioning System (GPS) time base, which may be synchronized with a Universal Coordinated Time (UTC) reference. GPS and UTC may differ by up to a few seconds to compensate for the number of leap year seconds corrections added to UTC since Jan. 6, 1980. BSs 106 are further required to radiate pilot PN code sequences within xc2x13 xcexcs of CDMA System Time and all CDMA channels radiated by BSs 106 are required to be within xc2x11 xcexcs of each other. The rate of change for timing corrections may not exceed xe2x85x9 PN chip (101.725 ns) per 200 ms.
Moreover, in accordance with IS-95 and IS-98 standards, each WD 110 is required to use a time base reference used to derive timing for the transmit chip, symbol, frame slot, and system time. During steady-state conditions, each WD 110 is also required to have a timing reference within xc2x11 xcexcs of the time of the earliest arriving multipath component being used for demodulation, as measured at the WD 110 antenna connector. In addition, if WD 110 time reference correction is needed, then it is to be corrected no faster than xc2xc PN chip (203.451 ns) in any 200 ms period and no slower than xe2x85x9c PN chip (305.18 ns) per second.
These stringent timing requirements are necessary because of the interdependence between BS 106 and WD 110 timing. FIG. 2 illustrates the timing relationship at various points within system 100. The start of CDMA System Time is Jan. 6, 1980, 00:00:00 UTC, which corresponds to the start of GPS time, indicated as GPS time stamp zero (GPS TS-0). Because, as noted above, each BS 106 time base reference is to be synchronized to CDMA system time, GPS provides an absolute time reference and each BS 106 transmission includes a GPS time stamp. For convenience, GPS TS-0 will be used heretofore to demonstrate the timing relationships between BS 106 and WD 110.
As indicated in FIG. 2, the interval denoted by reference numeral A1, demonstrates the trailing portions of the PN codes sequences conveyed by the pilot signals transmitted by BS 106 during forward link transmissions, prior to the start of CDMA System Time. The notation 0(n) denotes a portion of the PN code sequences, which comprise n consecutive zeros. The initial state of the long PN code sequence is configured with a xe2x80x9c1xe2x80x9d at the most significant bit (MSB), followed by 41 consecutive xe2x80x9c0xe2x80x9ds. Similarly, the initial state for both, the I and Q short PN code sequences are configured with a xe2x80x9c1xe2x80x9d at the MSB, followed by 15 consecutive xe2x80x9c0xe2x80x9ds.
Interval A2 demonstrates the beginning portions of the pilot PN codes sequences transmitted by BS 106 to WD 110 at GPS TS-0. It is to be noted that BS 106 is synchronized with the absolute time reference provided by GPS in order to transmit pilot signals at exactly 2 second intervals (i.e., even second marks). Even second marks are generally divided into twenty-five 80 ms. periods for CDMA frame boundary timing. Moreover, for the Paging Channel, Forward Traffic Channel, Reverse Traffic Channel, and Access Channel, the 80 ms. period is divided into four 20 ms. frames. For the Sync Channel, the 80 ms. period is divided into three ≈26.66 ms. frames. The pilot PN sequence repeats every ≈26.66 ms. and the ≈26.66 ms. frame boundaries coincide with the pilot PN sequence rollover points, which are offset in the forward CDMA channel to identify the transmitting sector of BS 106.
Interval B3 indicates the reception, by WD 110, of the pilot PN code sequences after a one-way forward link transmission delay (xcex94fl). The forward link transmission delay xcex94fl may include delays attributable to the line-of-sight (LOS) propagation delay (xcex94LOS) between BS 106 and WD 110, as well as BS 106 and WD 110 processing and hardware delays (xcex94bf, xcex94wf, respectively) associated with processing the forward link transmissions.
Interval C3 indicates that WD 110 aligns the timing of the reverse link transmissions with the timing of the received forward link transmissions. This may be achieved by taking into account the well-known forward link processing and hardware delays xcex94wf of WD 110, the well-known reverse link processing and hardware delays xcex94wr of WD 110, and compensating for the delays by advancing the timing of the reverse link transmissions to correspond to the forward link timing at the antenna connector of WD 110.
Finally, interval D4 indicates the reception, by BS 106, of the reverse link signals conveyed by WD 110, after a one-way reverse link transmission delay (xcex94rl). The reverse link transmission delay xcex94rl may include delays attributable to the line-of-sight (LOS) propagation delay (xcex94LOS) between BS 106 and WD 110, as well as BS 106 and WD 110 processing and hardware delays (xcex94br, xcex94wr, respectively) associated with processing the reverse link transmissions.
As noted above, the forward and reverse link WD 110 hardware/processing delays xcex94wf, xcex94wr are generally well-known and stable. In order to ensure that such delays are accounted for, WD 110 may be specifically calibrated in advance for such purposes. However, unlike xcex94wf, xcex94wr, BS 106 forward and reverse link hardware/processing delays xcex94bf, xcex94br are subject to change and may be difficult to measure. BSs 106 are not configured identically and, depending on traffic statistics, urban densities, frequent RF tuning, and system resources, each BS 106 may be equipped with a variety of system components, each of which have their own delay characteristics. Unless these delays are calibrated and compensated for, there is no certainty that signals at selected reference points have the requisite timing.
Considerable effort and human resources are required to determine and calibrate the delays due to BS 106 components. In many cases, services may be shut down and numerous technicians may be tasked to adequately assess and effect such calibrations.
Moreover, BSs 106 are frequently being modified and upgraded to provide better service or compensate for faulty equipment. Because each component, as noted above, manifests certain delay characteristics, each modification, whether it be a new cable, a new component, or antenna repositioning, requires the re-assessment and re-calibration of BS 106 delays.
Clearly, BS 106 delay determination and calibration is time and task intensive, requiring substantial economic and manpower resources. Accordingly, what is needed is a system and method for dynamically calibrating base station timing.
The present invention addresses the need identified above by providing a novel system and method capable of dynamically calibrating base station timing.
System and methods consistent with the principles of the present invention as embodied and broadly described herein include a base station for transmitting, receiving, and processing communication signals and a wireless communication device for communicating with the base station. The wireless communication device is configured to determine its location, to detect an arrival time of a first signal transmitted from the base station, and to calculate a line-of-sight delay corresponding to a line-of-sight distance between the wireless communication device and the base station. The line-of-sight distance is based on the base station location information and the wireless communication device location information. The base station measures a round trip delay corresponding to a delay incurred by the first signal and a delay incurred by a second signal transmitted from the wireless communication device back to the base station in response to the first signal. The base station then determines a base station timing calibration error based on the line-of-sight delay, the first signal arrival time, and the round trip delay, and dynamically calibrates the base station timing to compensate for the base station timing calibration error.